As more weapon and fire-control systems become dependent on GPS (Global Positioning System) for their accuracy and effectiveness, it becomes important for GPS receivers to be able to withstand RF (Radio Frequency) signal interferences, especially under a highly dynamic engagement scenario. The RF interference can adversely affect GPS receiver code and carrier tracking, resulting in degraded and unsatisfactory navigation performance.
In essence, a GPS system receiver determines its terrestrial location by triangulating its position relative to GPS satellites in orbit around the earth, by receiving signals transmitted from the satellites, measuring the travel times of the signals from the satellites to the receiver, and then calculating the distances of the satellites from the receiver based on the travel time. To measure the travel time, very accurate timing is necessary and, therefore, the GPS satellites carry atomic clocks. The receiver also needs to know the exact positions of the GPS satellites. In addition, for further accuracy the receiver compensates for atmospheric effects on travel time of the satellite signals to the receiver.
One basic function of a GPS receiver is to generate replica signals that can be correlated with the received satellite signals. Each GPS satellite can have a unique digital code sequence (e.g., a Pseudo Random Code) that is by analogy similar to a musical tune, so that the GPS receiver can distinguish signals from different GPS satellites. The GPS receiver knows the “tunes” or code sequences of the different GPS satellites, knows when the “tunes” are to be transmitted, and knows where the GPS satellites should be.
Upon receiving a code signal, the GPS receiver identifies the signal, generates a replica of the code signal, and seeks to “hum along” or synchronize the replica signal with the received code, and thereby track the received signal. This signal tracking includes two fundamental functions: code-phase tracking to track digital codes of received satellite signals, and carrier-phase tracking to track the carrier signals that are conveying the digital codes. When the receiver is receiving a code signal from a GPS satellite and the receiver's clock is synchronized with the clock onboard the satellite, then an amount of time that the receiver must delay the code replica signal to synchronize or correlate it with the received code signal is the amount of time it takes the received signal to travel from the satellite to the receiver. The receiver can use this time interval to determine a geographic distance between the satellite and the receiver. Signals from four or more different GPS satellites enable the receiver to synchronize its clock with the clocks onboard the satellites.
GPS satellites operated by the U.S. military transmit two different signals on two different carrier frequencies. The first carrier frequency, L1, has a frequency of 1575.42 MHz and carries two pseudo random digital codes as well as a status message (containing, for example, supplemental information regarding the satellite's orbit, the accuracy of its clock, and so forth). The first digital code on L1 is called a C/A (Coarse Acquisition) code. The U.S. military makes the C/A code for each U.S. GPS satellite known and available to the public sector. The C/A code repeats every 1023 bits, and modulates the L1 carrier frequency at a 1 MHz rate. The second carrier frequency, L2, has a frequency of 1227.60 MHz. In addition to the C/A codes transmitted on the L1 carrier frequency of the U.S. GPS satellites, a P(Y) code (“P” for precise, and “(Y)” when the code is encrypted) is also broadcast from each satellite on both the L1 and L2 carrier frequencies. The P(Y) codes are intended for exclusive use by the military. Each P(Y) code repeats on a 7-day cycle and modulates both the L1 and L2 carrier frequencies at a 10 MHz rate. Transmission of codes on two different carrier frequencies also allows military receivers to estimate atmospheric effects based on the different refractive effects that the atmosphere has on the two different carrier frequencies.
A problem with conventional GPS receivers is that if they are physically located near or otherwise are in the presence of interference (e.g., narrow band interference/jamming power), successful GPS tracking may not be possible or accuracy may be reduced. Prior attempts to address this problem included down converting the L-band GPS signal to a suitable frequency (IF) for digitization and then converting the GPS input signal to the digital domain. Then, while in the digital domain, the narrow band interference is removed using Fourier transformations or other equally complex techniques. For GPS anti-jam applications, once the narrow band interference is removed, the signal is converted back to an IF frequency, and then upconverted to L-band. Some systems bypass the IF conversion scheme and connect directly into the baseband processing chips of the receiver.
As is evident, a drawback to the above approach is that it adds significant complexity. First, the signal must be converted to the digital domain, which by itself is not significantly complex. However, the application of Fourier transformations adds significant complexity, as the calculations can be very intensive. Such complex calculations can consume significant processing power and/or require complex circuitry to implement.
Accordingly, there is a need in the art for a device and method for simply removing or minimizing the effect of narrow band interference/jamming power. This would enable a GPS receiver to accurately determine a position while the receiver is in the presence of jamming signals and/or narrow band interference.